The invention relates to waveguide lens antennas and in particular to means for increasing the bandwidth of such antennas and to design techniques that permit the fabrication of antennas having any desired degree of phase compensation.
Broadband wavelength lenses of the type comprehended by the invention are of primary interest for multiple-beam antennas operating at microwave frequencies. Multiple-beam antennas, in general, consist of an aperture (such as a lens or parabolic reflector) which focusses r.f. energy radiated by one or more elements in a feed array. Normally, a feed array consists of a large number of radiating elements, usually 19 or more.
Most types of r.f. focussing apertures, such as parabolic reflectors, reflectarrays, and certain types of lenses, commonly used for communications and radar applications, are not suitable for multiple-beam antennas. For instance, the normally-large array of feed elements result in excessive aperture blockage of center-fed parabolic reflectors. This blockage results in loss of efficiency and degradation of pattern shape. Parabolic reflectors with offset feeds do not suffer such blockage, but they do have very poor beam scanning characteristics and are hence undesirable for multiple-beam antenna application. Reflectarrays, which are reflecting arrays of elements which focus energy from one or more broad-beam feed elements, have the same general weaknesses as parabolic reflectors. Luneburg lenses and bootlace lenses have good bandwidth and beam scanning characteristics; however, they have poor physical characteristics, such as excessive weight and structural complexity. Others, such as waveguide lenses of previous design, have good physical characteristics but poor electrical characteristics (such as limited bandwidth). In fact, there is no r.f. focussing aperture currently available that is completely satisfactory for multiple-beam antennas operating over the X-band communications band.
Accordingly, there currently exists the need for a broadband waveguide lens that offers substantial improvement over those previously available. It is desirable that such a lens make possible the achievement of certain important capabilities from multiple-beam antennas having simple, lightweight structures. One such capability is the formation of nulls in broadcoverage patterns (formed by turning on many single contiguous beams). Such nulls must be well-shaped and capable of being formed and maintained over a substantial band-width. The present invention is directed toward providing and improved broadband waveguide lens antenna having such a capability.